1. Field of the Invention
This invention relates to heating circuits and particularly, but not exclusively, to heating circuits suitable for heating electrically heated blankets or pads.
2. Description of the Prior Art
It is known to heat an electric blanket or pad by means of a heating circuit comprising an elongate heating conductor and switch means (for example a thyristor or a mechanical switch) connected in series with the heating conductor between input terminals for connection to an electrical supply, in particular an AC supply, switch control means (for example an electronic control circuit or a bimetallic strip) operative to close the switch means to cause current to flow from the supply through the heating conductor, an elongate sensor conductor spaced from the heating conductor, and temperature sensitive means disposed between the heating and sensor conductors whereby its impedance drops in the event of overheating. Any such drop in impedance can be sensed and used to disable the supply of current to the heating conductor.
The current through the heating conductor may be DC (for example half-wave rectified AC if the switch means is a single thyristor) or it may be AC. If the current is AC, then while the blanket is calling for heat AC current will continually flow through the heating conductor. If overheating occurs, while it is generally possible to detect this if the overheating is general, that is to say if it occurs along the length of the temperature sensitive means, a problem arises if, as is likely, the overheating is localised at a position along the length of the temperature sensitive means. The reason for this is that, since current continuously flows through the heating conductor when the switch is closed, there is a voltage gradient along its length. Thus, if the sensing of overheating is conducted by monitoring the potential on the sensor conductor due to the localised impedance drop, the potential will vary substantially in accordance with the location of the overheat and it therefore becomes difficult to detect overheating at certain positions.
Naturally, it has in the past been thought essential to monitor the impedance whilst the switch means is closed since only then, of course, will overheating occur. The approach to solving the above problem in the past, as can be seen from UK Pat. No. 793,783 and US Pat. No. 2,831,099, has therefore been to evolve circuits for monitoring the impedance of the temperature sensitive means whilst the switch means is closed.